Square wave generator



May 5, 1959 J. B. SPELLER SQUARE WAVE GENERATOR Filed oct. 26. 1954 IN V EN TOR.

'JZJCK B. SPE-LEP L'SQnm/M HTOP/VEY United States atent O SQUARE WAVE GENERATOR Jack B. Speller, White Plains, NX., assignor, by mesne assignments, to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Application October 26, 1954, Serial No. 464,837

1 Claim. (Cl. Z50-27) My invention relates to a square wave .generator and more particularly to an improved square wave generator which substantially eliminates the defects of the prior art. My improved square wave generator produces a square output wave form which approaches the ideal form.

In many pulse circuits itis desirable that a square wave form be generated from a sinusoidal input signal. This output wave form may be employed in a sawtooth generator to produce an accurate sweep signal. The leading or trailing edge of the wave may be used to trigger another circuit. Such square waves may also be ernployed in counters. It will readily be appreciated that where such a square wave is employed to trigger other circuits or to generate an extremely accurate sawtooth, it is necessary that the leading and trailing edges of the wave be sharp. Further, it is desirable that the wave be symmetrical and that it cross the time axis at times corresponding to the times when the sinusoidal input signal crosses the time axis.

In the prior art, in order to generate a square output wave form from a sinusoidal input signal, various circuits have been employed. Clipping circuits have been used to clip the peaks from a sinusoidal input signal to produce a somewhat rectangular wave. It will readily be appreciated that the resulting wave is not, however, truly rectangular since a portion of the sinusoidal wave remains in the output.

Another means which has been employed in the prior art to approximate a square or rectangular wave form is the multivibrator circuit. As is known in the art, the multivibrator circuit employs feed back connections each including a large capacitor coupling the plate of one of the tubes employed in the circuit and the grid of the other tube. The multivibrator circuit, however, does not produce a truly square or rectangular output wave form. While the output wave form of such a circuit has a sharp trailing edge, the leading edge represents an exponential rise of the output voltage Afrom its initial value to a maximum value. This phenomenon results from the fact that the feedback capacitor, coupling the plate of the tube 'whose condition is being reduced to zero to the grid of the tube whose conduction is being increased from zero to a maximum, must be charged from an initial value to a final value before the plate output wave form of the tube whose conduction s being reduced can reach its maximum. Since the feed back coupling capacitors are large, the time constant of the charging period is long and the leading edge of the output wave form rises exponentially to attain its maximum value only after a period of time. It is to be noted also that the output wave form from the multivibrator circuit will not cross the time axis at times corresponding to the times when a sinusoidal input signal crosses the time axis. This is true since either tube will conduct as soon as its grid-to-cathode voltage reaches the cut-oli value of grid potential and will not be cut oi until its grid-to-cathode signal again. drops to cut-olf 2,885,549 f Patented May 5, 1959 potential. The resulting output wave is not, therefore, symmetrical.

Another means which has been employed in the prior art to approximate a square output wave 4form is the multiar circuit, which employs a single tube. This circuit includes a regenerative lfeed back means to produce, from a sinusoidal wave, an output wave form which approximates a rectangular or square shape. Since this circuit employs a regenerative yfeed back action, it produces an output wave form having a sharp leading edge. The input grid circuit of the multiar includes a seriesconnected crystal which prevents a positive input signal Ifrom affecting the plate current. The lgrid-to-ground circuit employs a capacitor for preventing oscillations. When the input signal to the circuit drops to cut-olf potential during the negative half cycle of an input wave, the tube cuts off abruptly to produce a sharp wave front in the output. During the period of time when the input wave goes from cut-ott to negative maximum, the grid-to-ground capacitor acquires a charge of a polarity which tends to keep the tube at cut-olf. When the input signal vgoes `from negative maximum toward cut-cfr", the tube will not again conduct its normal plate current until the grid-to-ground capacitor has lost the charge acquired during the period when the input signal Went from cut-oit to negative maximum. As a result, the trailing edge of the output wave form represents an exponential drop from a maximum value to the value at cut-o. This multiar arrangement has an additional disadvantage in that its duty cycle varies as the signal size varies, since the tube will not conduct until the input sine wave reaches cut-oli voltage. 1

I have invented an improved square wave generator which substantially overcomes the defects of the square wave generators of the prior art to produce an output square wave which is symmetrical and which approaches the ideal form.

One object of my invention is to provide an improved square wave generator which substantially eliminates the defects of generators of the prior art to produce an output wave form which approaches the ideal form.

Another object of my invention is to provide an improved square wave generator to produce an output wave form which crosses the time axis at points substantially corresponding to the times when a sinusodial input signal passes through zero.

A further object of my invention is to provide an improved square wave generator to produce an output wave form in which the trailing edge of the wave is nearly vertical.

Other and further objects of my invention will appear from the following description.

In general my invention contemplates the provision of a pair of electron tubes connected in push-pull relationship. Each of the individual tubes is provided with a regenerative feed-back means and arranged to operate in a manner similar to a single tube multiar. The tubes are also interconnected by multivibrator connections. Since the rise and fall times in the output waveform are determined by the characteristics of the feed-back means, I may eliminate from the multivibrator connections the large feed-back coupling capacitors of multivibrators of the prior art. If desired, small feed-back coupling capacitors may be employed. The multivibrator connections provided enable me to eliminate the grid-to-ground capacitors employed in multiar circuits of the prior art. It will be appreciated that my square wave generator produces yan output waveform which very nearly approaches the ideal form. It has both a sharp leading edge and a sharp trailing edge. Since I employ a pushpull connection the output wave form from my square assume wave generator crosses the time axis at times substantially corresponding to the times when a sinusoidal input signal crosses the time axis. The output wave form Afrom my square wave generator is, consequently, symmetrical. I provide my square wave generator with means for compensating yfor differences between the characteristics of the respective tubes employed therein.

The accompanying drawing which forms part of the instant specification and which is to be read in conjunction therewith is a schematic view of my improved square wave generator.

More particularly referring now to the drawing, my improved square Wave generator includes a pair of electron tubes and 12. The tubes 10 and 12 may be triodes but preferably I employ pentode tubes as shown because of their numerous known advantages over triodes. As can be seen by reference to the figure, the tubes 10 and 12 are connected in push-pull relationship. The input circuit of my generator includes an input transformer, indicated generally by the reference character 14. The transformer 14 includes a primary winding 16, one side of which is connected to ground by a conductor 18. The sinusoidal input signal to the circuit is impressed on a pair of terminals and 22 of the primary winding 16. The respective cathodes 24 and 26 of the tubes 10 and 12 are connected by respective conductors 28 and 30 to a line 32 -connected to a center tap 34 on the secondary winding 36 of the transformer 14. The respective sides of the transformer secondary winding 36 are connected to the respective control grid input circuits of the tubes 10 and 12 by conductors 38 and 40. A pair of capacitors 42 and 44 are connected between the respective conductors 38 and 48 and the line 32 to prevent oscillations.

The control grid input circuits to the respective grids 46 and 48 of the tubes 10 and 12 include respective blocking crystals 50 and 52. The respective plates 54 and 56 of the tubes 10 and 12 are connected to an appropriate positive potential source such as a battery terminal S8 by plate resistors 60 and 62 and the line 64. As is conventional in the case of pentodes, the respective screen grids 66 and 68 of the tubes 10 and 12 are connected to the line 64 through screen grid resistors 70 and 72. Similarly, the respective suppressor grids 74 and 76 of the tubes 10 and 12 are returned to the cathodes 24 and 26. The output Wave form of my improved square wave generator may be taken from a pair of terminals 78 and 80 connected to the respective plates 54 and 56 by conductors 82 and 84.

In order to provide a control grid bias for tube 10, I connect the control grid 46 of the tube 1i) to a voltage divider 86 by a resistor 88. Similarly, the control grid 48 of tube 12 is connected to the divider 86 by a resistor 90. The brush 91 of the voltage divider 86 is connected to ground. The respective plates 54 and 56 of the tubes 10 and 12 are connected to respective control grids 48 and 46 by ,resistors 92 and 94. YIt will readily be appreciated that resistors 60, 92 vand 90 and the portion of the voltage divider 86 to the right of brush 91 in the drawing provide a bias for the control grid 48 of tube 12. Similarly, resistors, 62, 94 and 88 and the portion of the voltage divider 86 to the left of brush 91 provide a bias for the control grid 46 of tube 10. Brush 91 may be moved along the divider 86 to adjust the relative magnitudes of the bias on grid 46 and the bias on grid 48. This adjustment may be employed to compensate for differences in characteristics between tubes 10 and 12. If desired, small feed-back coupling capacitors 93 and 95 may be connected in parallel with the respective resistors 92 and 94 to .ensure rapid rise and fall times. It is to be understood that capacitors 93 and 95 may be eliminated entirely, since y the rise and Vfall times Iare controlled primarily by feedback means, to kbe described in detail hereinafter. In any case, the size of these capacitors is not suiiciently large to aifect the output wave form to any appreciable degree.

Line 32, which is connected to both cathodes 24 and 26, is connected to ground through a cathode resistor 96. It will be appreciated, as will be explained in detail hereinafter, that the circuitry thus far described is a pair of electron tubes connected in push-pull relationship and arranged to operate as a bistable, cathode-coupled multivibrator. i

vIn order to ensure s harp rise ytimes for the output wave form from my improved square wave generator, I dispose feed-back transformers, indicated generally, respectively, by the reference characters 98 and 100, in the respective control grid input circuits of the tubes 10 and 12. These transformers 98 and 100 employ a step-up ratio from cathode to grid to increase the effect of changes in plate current in the respective tubes. They are wide band transformers which have little inductance and low interwinding capacity to ensure fast rise and fall times. The primary winding 102 of the transformer 98 is disposed in the line 28 providing the cathode-toground connection for tube 10. The `secondary winding v104 of transformer 98 is disposed in the line 38 providing the connection between secondary winding 36 of transformer 14 and the control grid 46. The primary winding 106 of transformer 100 is disposed in the line 30 providing a connection between the cathode 26 of the tube 12 and the line 32. The secondary winding 108 of transformer 100 is disposed in the line 40 providing a connection between control grid 48 of tube 12 and the secondary winding 36. If desired, respective crystals 110 and 112 may be connected in parallel with the secondary windings 104 and 108 to damp any oscillations which might result from the feed-back transformers.

It is to be understood that in the quiescent condition when no signal input is impressed on the terminals 29 and 22, the circuit functions as a multivibrator; that is to say, in this condition one of the tubes 10 or 12 is normally conducting and the other of the tubes is normally off. This condition prevails until such time as an input signal of a polarity to disturb the condition of the circuit is applied. Let us assume, for example, that tube 12 is a normally conducting tube and tube 10 the normally nonconducting tube. If an attempt is made to apply a positive signal to the control grid 48 of the tube 12, no change in the operating conditions results, since the positive input signal is blocked by the crystal 52. If, however, a negative signal is applied to the control grid 48 through the crystal 52, it tends to reduce the plate current owing through the tube 12. rlhis drop in plate current through the tube 12 results in an increase in potential of the plate 56. The connection provided by the resistor 94 and capacitor from the plate 56 to the grid 46 causes grid 46 to rise. When grid 46 thus rises, tube 10 begins to conduct with the result that the plate potential at plate 54 drops. This drop in potential of plate 54 is .transferred to control grid 48 to reduce further the plate current iiowing through tube 12. When tube 10 thus starts conducting, the potential drop across the cathode resistor 96 rises thus further reducing the grid-to-cathode potential of tube 12 to cut down the plate current through tube 12 still more. It will be appreciated that this cycle is repeated and the action is cumulative so that the transfer of conduction from tube 12 to tube 10 is extremely abrupt.

A decrease in plate current owing through the tube 12 is lreflected into the input circuit of grid 48 by transformer in order to drive the grid 48 further negative and lfurther reduce the plate current. This regenerative feed-back action of transformer 100 together with the multivibrator action provided by cathode resistor 96 and by the connections between the respective plates 54 and 56 and grids 48 and 46 results in an extremely rapid transfer of conduction from tube 12 to tube 10. That is, the cutting ot of the normally conducting tube 12 is very sharp so that a nearly vertical leading edge in the output plate waveform of tube 12 results. Irt will be understood that when input signal to tube is on the negative half cycle while the input signal to tube 12 is on the positive half cycle, the action just described is reversed in that a transfer of conduction from tube 10 to tube 12 takes place.

In operation, when my improved square wave generator is employed to generate a square wave from a sinusoidal input signal, the sine wave is impressed on the terminals 20 and 22. The input wave produces a pair of grid signal waves for the respective tubes 10 and 12 which are of equal magnitude but 180 degrees out of phase with one another. It can be seen, therefore, that while the input signal to the tube 12, for example, is on the positive half cycle, the input signal to the tube 10 is on the negative half cycle. During this period, the conduction of the normally conducting tube 12 will not be affected and the tube 10 will continue at cut-off, since the input signal to the grid is on the negative half cycle. During the next half cycle of the input signals, however, the conditions are reversed and the negative input to the grid circuit of tube 12 tends to reduce the plate current thereof while the positive half cycle signal applied to the grid circuit of tube 10 will not affect the operation of the tube. As explained hereinbefore, however, a rapid transfer of conduction from through tube 12 to through tube 10 takes place. Tube 10 then conducts until the succeeding half cycle of the input signal. It is to be noted that my push-pull and multivibrator connections ensure that the output wave form of the circuit crosses the time axis substantially at a time when the input signal is zero. The input signal to the tube need not reach cutoff before a transfer of conduction takes place. It will occur when the signal input to the grid of the conducting tube changes polarity from positive to negative. It will be appreciated also that I have eliminated the grid-to-ground capacitors which are conventional in single tube squaring circuits of the prior art. Consequently, fthe resultant distortion of the trailing edge of the square wave output is eliminated. This occurs because transfer of conduction from one tube to the other occurs with the same rapidity ir-respective of which tube is on.

In order yto ensure that in the quiescent condition my multivibrator circuit is stable and will not be affected by differences between the characteristics of the respective tubes 10 and 12, I provide the voltage divider 86. Brush 91 may be moved in either one 4direction or the other to ensure a symmetrical output if the tubes are not identical. It will be appreciated that adjustment of brush 91 regulates the relative magnitude of the bias on tube 10 with respect to the bias on tube 12.

It is to be understood that the resistors 92 and 94 and 88 and 90 are sufficiently large that it can be safely assumed that they bleed a negligible current from the supply in comparison with the circuit as a whole. The values of these resistors may be calculated for a known grid swing to design the multivibrator. The amplitude of the output wave form swing obtainable at either plate may vary from a few volts to a few volts short of the voltage supplied by battery terminal 5S. At any level of swing, however, the rise time is constant. The necessary equations for designing the multivibrator may easily be obtained by considering the circuit in its quiescent condition.

Let

R1=the resistance of each of the resistors 92 and 94 R2=the resistance of each of the resistors 88 and 90 (including the respective portions of divider 86) Rs=the resistance of each of the screen grid resistors 70 and 72 RL=the resistance of each of the plate resistors 60 and RK=the resistance of cathode resistor 96 I1=the plate current through resistor 60 I2=the plate current through resistor 62 I1l=the screen grid current through resistor 70 I2'=the screen grid current through resistor 72.

'Ebb

Similarly, the grid-to-cathode voltage En of tube 10 may be written:

It will readily be appreciated that since the circuit is symmetrical and the components on either side thereof are as nearly identical as possible, Equation` 1 for E02 represents the grid voltage of either tube in the full on condition and Ecl of Equation 2 represents the grid voltage of either tube when in a nonconducting condition. The resistance RS is selected so that the screen dissipation rating of the tube is not exceeded. Subtracting Equation 2 from Equation 1, we obtain:

It is obvious that if tube 12 is normally conducting so las to be full on or nearly so, when a switching of conduction occurs I2 changes from full on to zero. The plate voltage swing S may, therefore, be represented by:

Assuming Ebb is known and since the grid of each tube follows the plate of the other tube, the design equations for the multivibrator may be obtained from Equations 1 to 4. Substituting S for lzRL in Equation 3 and solving for A, I obtain:

EaZ-Ecl 5) S- If the required swing S is known, A may easily be calculated, since Ecz is taken as zero while Ecl is generally taken to be a few volts more negative than cut olf for a given plate voltage on a particular tube. From Equation 1 I may write:

Knowing (I2-|I2,)RK, Ebb, and S, the value of Ebg may be obtained from Equation 7. With the known values of Ebg and E62, the corresponding values of I2 and I2' may be obtained from the tube characteristic curves. From Equation 4 I may write:

S 8 RL I2 Where I2 is the value determined from Equation 7. Rg may then be obtained from the relationship:

where (I2-Hy) RK is determined from Equation 6 and I2 and I2 are the values determined from the tube characteristic curves.

Also:

do) (l where R2 is large with respect to RL. The values of the resistors RL, R2, and R1 may ready be obtained from Equations 8 to 10 so thatV for a Vknown grid swing, battery voltage Eb,I and given tube characteristics an appropriate circuit may be designed.

Thus it will be seen that I have accomplished the objects of my invention. I have provided an improved square wave generator which produces an output square wave approaching an ideal wave form. I have, moreover, eliminated the distortion occurring at the trailing edge of the wave in multiar square wave generators of the prior art. Likewise, I have overcome the disadvantage of slow rise times which exists in multivibrator square wave generators of the prior art. Further, my improved square Wave generator produces an output wave form which crosses'the time axis at points corresponding -to the times when the input grid signal is zero. I have provided my improved generator with means for compensating for differences between the tube characteristics of the tubes employed in the circuit.

It willbe understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is therefore to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

A square wave generator including in combination a -pair of electron tubes each including a plate and a control grid and a cathode, respective control grid input circuits for said -pair of tubes, means for supplying said control grid input circuits with respective sinusoidal -input signals which are out of phase, means interconnecting said electron tubes Ato operate as a multivibrator and respective regenerative feedback circuits associated with said tubes, each of said regenerative feedback circuits including a transformer having a primary winding and a secondary winding, means connecting one terminal of the primary winding of each of said transformers to a respective one of said cathodes, means connecting the other terminals of said primary windings to ground and means connecting the secondary winding of each of said transformers between said means for supplying said input signals and a respective one of said control grids, the arrangement being such that the primary and secondary windings of a transformer are connected to the respective cathode and control grid input circuit of the same tube.

References Cited in the tile of this patent VUNlTED STATES PATENTS 2,254,852 Miller Sept. 2, 1941 2,426,996 Goodall Sept. 9, 1947 2,675,476 Isberg Apr. 13, 1954 2,701,311 Gray Feb. 1, 1955 2,748,272 Schreck May 29, 1956 2,782,309 Aasma Feb. 19, 1957 FOREIGN PATENTS 135,254 Australia Ian. 12, 1949 873,527 France July 10, 1942 OTHER REFERENCES Waveforms by Chance et al., Radiation Laboratory series, published by McGraw-Hill, pp. 230-231. 

